1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and a computer program product.
2. Description of the Related Art
Research is being carried out worldwide on a technique to form an image by removing external light even when a photogenic subject is illuminated in an environment that includes the external light. Conventional image forming methods that employ the above technique are disclosed in Japanese Patent Laid-Open Application Nos. 2002-10030 and 2003-187230. In these methods, in the environment that includes the external light, an image of the photogenic subject is taken by switching off the illumination to acquire an image without illumination, an image of the photogenic subject is taken by switching on the illumination to acquire an image with illumination, and a luminance value of each point of the image without illumination is subtracted from a luminance value of each point of the image with illumination to form an image without the external light.
The conventional technique is explained in detail with reference to FIG. 19. As shown in FIG. 19, an image processor subtracts a luminance value 2c of a point 2b of an image without illumination 2a from a luminance value 1c of a point 1b of an image with illumination 1a to form an image 30a having a luminance value 30b. By carrying out a similar process for each point, the image processor forms the image without the external light. Alternatively, a light modulation method is known that carries out the method at hardware level inside an imaging device.
However, in the conventional technique, if the external light is strong, an image having an accurate luminance value of the illumination cannot be formed. In other words, because the luminance value of the image includes an upper limit, if the external light is strong, i.e., exceeds the upper limit, all the luminance values become equal to the fixed upper limit, and the image processor subtracts the luminance values of the external light from the upper limit instead of subtracting the luminance values of the external light from the original luminance values. Due to this, the luminance values of the illumination are eliminated at some points and the image having the accurate luminance value of the illumination cannot be formed.
This problem is explained in detail with reference to FIG. 20. As shown in FIG. 20, for example, if the external light is strong, the image processor subtracts a luminance value 2d of the point 2b of the image without illumination 2a from an upper limit 1e instead of subtracting the luminance value 2d from an original luminance value 1d of the point 1b of the image with illumination 1a. Thus, an image 40a having an inaccurate luminance value 40b is formed. Moreover, in the conventional light modulation method, the structure of the imaging device becomes complicated and costly.